Cassava, Uganda’s major staple after matooke, feeds over half a billion people in the world with Africa contributing to about 50% of the global production. It is a reliable source of carbohydrates. In prolonged drought period, cassava stands tallest above all other staples in tolerating stress from the sun making it Uganda’s most reliable food security crop. Often referred to as a poor man’s food, cassava is steadily finding its way onto a rich man’s plate, assuring the poor of its reliability in times of need.
But the workhorse crop is threatened, and it appears that only the only solution rests with advanced biotechnology to create virus resistant transgenic varieties.
Cassava (Manihotis esculenta), a shrub belonging to the family Euphorbiaceous, develops a beautiful green leafy crown averaging 3 meters above ground. Its stems have the characteristic rough nodes that the short-lived leaves leave behind as they drop off. The normally dark green leaves open their lobbed palms at the end of every slender petiole as to tap the maximum photo energy for making the excess nutrients it stores in its swelling tubers. The tuber is roasted, boiled, fried, eaten raw, and sundried. The delicious leaves that contain protein, iron and B vitamins are grazed on by some communities.
During extended drought, cassava shades off the burden of its leaves and enters into temporary dormancy; the stem is corky and does not require much water to sustain it, this inactivity implies, that the plant is not in a rush to convert stored starch to sugar for its utilization, making it a sustainable source of starch.
Cassava compared to other staples
Uganda’s major staple, banana, is supported by extended leaves that form its pseudo stem. The pseudo stem relies heavily on water to form its support function. During extended drought, it struggles to remain firm as a result of the drastic reduction of water in its cells that causes it to easily crumble under the weight of its fibers and extravagant leaves. When it falls, it falls with its fruits.
Sweet potato, another root crop commonly grown in Uganda, is so vulnerable to drought, aside the weevils that find refuge it its sweet tubers, it quickly goes bad and less desirable to consumers. The leaves and vines dry up and when it rains, the crop tends to regenerate directly from tubers using the stored food as a direct supply for the new shoots making the tuber less palatable and in some cases unyielding to cooking.
Cassava tastes sweet to humans as it does to pests and viruses. Mites, whiteflies and mealy bugs feed on it. The viruses residing in cassava take a free ride on whiteflies as they fly about feeding from plant to plant, and in the process spreading viruses. The virus that attack cassava are the Cassava Mosaic virus and Cassava Brown Streak virus.
The brown streak currently poses the greatest threat with yield losses up to 100%. It spares the leaves of its beauty but attacks the heart, the tuber making it unfit for food or feed. A farmer continues to expend labor in tending the crop, when harvest time comes; the tubers are corky, constricted, black and rotten. It cannot be eaten and neither can it make Waragi, a locally made distilled beverage that perhaps could take away the worry of the pending food shortage.
Globally, Cassava Brown Streak Disease (CBSD) has been recognized as one of the seven most dangerous plant diseases for its impact on food and economic security. It causes yield loss up to 100 percent because of lesions in storage roots. There are few sources of tolerance to the viruses and it is spreading rapidly throughout East Africa. The virus is transmitted from one crop to another by whiteflies.
The symptoms of CBSD are: dark brown “streaks” and “spots” on stems, with dead spots on leaf scars, these streaks are most prominent on upper, green portions of the stem; The disease may cause cracks and discoloration in the storage roots; It often causes root constriction and malformation; the harvested roots have corky, yellow-brown necrotic spots
CBSD spreads mainly through planting cassava cuttings from infected cassava plants. This also explains the rapid spread of the disease in areas where it is re-emerging, such as Uganda. Additionally, sharing and distribution of infected planting materials is responsible for rapid spread of the disease. It is also spread by white flies; Cassava brown streak disease also spreads through multiplication centers if the original source of cassava materials were infested or if the planting materials being distributed are not checked for the presence of CBSD. Planting of susceptible varieties helps build up CBSD in the affected countries.
Control CBSD by uprooting and burning all the affected plants as soon as you observe disease symptoms; plant CBSD tolerant varieties like Nase 3, Nase 14, Nase 15, Nase 18, and Nase 19; Plant materials from certified fields.
RNAi or gene silencing to produce resistant transgenic cassava varieties
Virus Resistant Cassava for Africa (VIRCA) project is in the process of developing transgenic farmer preferred cassava varieties which are resistant to both viruses. This is a collaborative effort involving Uganda’s National Crops Research Resources Institute (NaCRRI), Kenya’s Agricultural Research Institute (KARI), and Donald Danforth Planter Center in Missouri. These varieties when actualized will go a long way to ensuring food security and livelihoods of several thousand households who depend on cassava and whose fields are currently being devastated by CBSD. Resistant varieties are the much needed solution if this climate smart staple is to continue providing carbohydrate during prolonged droughts.
A collaborative research effort between NARO, Kenya Agricultural Research Institute in partnership with Donald Danforth Plant Center through the Virus resistant Cassava for Africa Project is applying modern biotechnology termed Gene silencing to silence the expression of brown streak in the cassava plant. Scientists hope by the time the crop becomes ready for farmers the activitists who are always quick to criticize any GM product will not be in a position to block regional food security.
Isaac Ongu is an agriculturist, science writer and an advocate on science based interventions in solving agricultural challenges in developing countries. Follow Isaac on twitter @onguisaac.